Effector T lymphocytes recognize antigens in association with a product of the major histocompatibility complex (MHC) through the T-cell receptor for antigen (TCR), a heterodimer composed of disulfide-linked alpha and beta chains. The appropriate pool of alpha/beta TCR-bearing cells is generated in the thymus upon positive and negative selection, a process which eliminates TCRs with a high affinity for self MHC alone while expanding T cells bearing TCRs with affinity for self in association with foreign antigens. Although some progress in understanding this process has occurred, the precise mechanism of thymic education remains poorly understood. In addition to the alpha/beta TCR, a second TCR, termed gamma/delta, has been described on the surface of a small subset of cells and thymocytes. The function and the ligand(s) of the gamma/delta TCR remains unclear, although cells using this receptor have been associated with MHC-restricted and non-restricted cytotoxicity. Because of its expression of immature T cells and thymocytes, it has been proposed that the gamma/delta TCR may play a role during the early events of thymic T cell development, including thymic education and T cell tolerance induction. This project aims to define the function, ligand(s), and ontogeny of expression of the gamma/delta TCR both murine and human models. In order to dissect this project at the molecular level, we have first sought to define the complexity of the variable (V), diversity (D), and joining (J) element usage for the newly-described human delta TCR gene, because these elements combine to form the ligand binding part of the receptor. We analyzed the gene arrangements associated with the newly described delta TCR gene from a series of 19 consecutive precursor T-cell (lymphoblastic) neoplasms that represent discrete stages surrounding the TCR gene rearrangement process. Significantly, the delta TCR gene showed rearrangement in most (13/19) of these T-cells, and in addition, it was rearranged in two cells displaying no rearrangement for any other TCR gene. While only a single, productive V5-J8-C5 joining has previously been identified in man, our survey revealed two additional types of delta gene rearrangements associated with cell-surface TCR expression that presumably represent novel V gene usage. This analysis demonstrates 1) a major subclass of human precursor T-cell neoplasms belonging to the gamma/delta T-cell receptor-rearranging subtype 2) a narrow repertoire of human V5 gene usage and 3) the utility of delta gene rearrangements as a diagnostic clonal marker in precursor T lymphoblastic neoplasms. There are now a total of six V genes, three D genes and one principal J gene known to contribute to the complexity of the human gene. A genomic clone was derived for one of the new V genes, while the other V gene rearrangement was not detected by the J delta 1 probe, consistent with the possibility that this cell uses a J delta segment distinct from the one analyzed here, such as J delta 2. Genomic clones for the D & J regions are being developed to study the ontogeny of the gene's expression in human thymus. In addition, a series of precursor B cell leukemias was analyzed for rearrangements at the delta chain gene. Human precursor B cell acutelymphoblastic leukemias (ALL) have previously been known to frequently rearrange their TCR alpha, beta and delta genes. We found that the majority of precursor B cell ALL (12 of 16) showed rearrangement or deletion of one or more TCR delta genes in contrast to mature B cell neoplasms in which no TCR delta gene rearrangements were detected.